This invention relates to lithium cells, and more particularly, to lithium cells having a low internal resistance and improved shelf life, and additionally exhibiting improved cycle performance when constructed as secondary cells.
Because of their high voltage and capacity, an increasing demand is now imposed on lithium cells in a variety of applications covering from memory backups for personal computers and video cassette recorders to drive sources for cameras or the like. The improved features of lithium cells including high voltage and capacity are attributable to the use of lithium as negative electrode active material. However, lithium is so active and thus unstable that it immediately reacts with other materials as is well known in the art. For this reason, the electrolyte solution used in conventional lithium cells is an organic electrolyte solution which is generally believed to be relatively stable. Nevertheless, it is inevitable that the lithium negative electrode react with organic electrolyte solution during actual service of the cell, resulting in losses of various properties.
More particularly, primary lithium cells suffer from the following problem. Although the primary cell experiences only a discharge process, the discharge process produces active lithium ions which react with the electrolyte solution to form by-products. As the cell is used or shelf stored for a longer time, the cell increases its internal resistance, which disturbs discharge more or less. A more complex problem occurs with secondary lithium cells which involve a charging process in addition to a discharging process as in the primary cells. Probably because of the unevenness of pre-existing by-products, charging incurs uneven electrodeposition of lithium which grows as so-called dendrites, resulting in such failures as lithium removal and cell internal shortcircuiting.
In the state of the art, lithium must be processed before it can be used as cell electrodes, particularly as secondary cell electrodes. Several attempts have been made to solve the above problems by reducing the reactivity of lithium, for example, by alloying lithium with another metal or metals to form a lithium alloy or combining lithium with a carbon substrate to form a lithium-carbon composite material. These attempts provide more or less improvements, but are not fully successful. Particularly for secondary cells, it is desired to solve the problem that their performance is deteriorated with repetition of charge/discharge cycles.
Methlie, U.S. Pat. No. 3,415,687 discloses a cell comprising an electrolyte containing less than 500 ppm, preferably less than 150 ppm of water. Daifuku et al., U.S. Pat. No. 4,717,634 discloses a cell comprising a positive electrode containing as an active material a polyaniline having a water content of 10,000 ppm or less, preferably 1,000 ppm or less, a negative electrode containing lithium, and a non-aqueous electrolytic solution.